Kinetics and Mechanisms of Substitution Reactions on Decacarbonylmanganeserhenium

The kinetics of substitution reactions on decacarbonylmanganeserhenium, MnRe(CO)io, are reported for the entering ligands P(C6H5)3, P(C4H9)3 and P(OC6H5)3. Synthesis of all products and careful infrared analysis have allowed complete product assignment. For all three ligands the rhenium-substituted product, (CO)5MnRe(CO)4L, predominates, although formation of the bis-substituted product, L(CO)4MnRe(CO)4L, can be quite significant depending on the reaction conditions. A very small amount of the manganese isomer, L(CO)4MnRe(CO)s, was formed early in the reaction. In no case were the homodimetallic species (Mn2(CO)10 or Re2(CO)10) or substituted derivatives of the diatomics observed. The manganese isomer fails to rearrange to the rhenium isomer under the conditions of the kinetics reaction. The rate constants and activation parameters are nearly identical for the three ligands. These data strongly suggest that CO dissociation is the rate-determining step and are inconsistent with cleavage of the manganese-rhenium bond. Our proposed mechanism involves CO dissociation from the manganese center and an intermediate/transition state with carbon monoxide bridging the manganese and rhenium.